The art of printing images with micro-fluid technology is relatively well known. A disposable or (semi)permanent ejection head has access to a local or remote supply of fluid (e.g., ink). The fluid ejects from an ejection zone to a print media in a pattern of pixels corresponding to images being printed.
Accurately knowing the amount of fluid available for use during printing lends itself to a variety of consumer features. Imaging devices can warn users of impending depletion of fluid. Users can re-supply fluid to prevent voiding warranties. Imaging can cease to avoid de-priming ejection heads, etc. Manufacturers have implemented a variety of fluid measurement sensors and techniques. Each has its own set of advantages and problems. Some are cheap while others are costly. Some work as intended while others have proven so poorly that users regularly ignore them. Still others are complex, including complicated processing algorithms. The optimum balance is to provide accurate fluid level measurement over a lifetime of a supply item, but without adding complexity or cost. Some of the more popular strategies in the art contemplate float sensors, magnetic sensors, torques sensors, optical sensors, valves, fluid drop-counting, electrical probes, capacitance determinations, or the like.
With capacitive style fluid detection, it is common to fashion two metal plates (electrodes) with spacing between them. Upon application of electrical energy, circuitry measures capacitance of the media (e.g., fluid) residing in the spacing. The amount of capacitance varies according to the amount of the media and level detection is made possible. The plates reside wholly within the fluid or external to a housing containing the fluid. Alternatively, one plate resides in the fluid while the other resides out of the fluid. Spacing between the plates, sizes and shapes of the plates and material selection are just some of the many design options. Pros and cons dictate the choices.
In any design, capacitance detection has inherent drawbacks making them dubious for micro-fluid applications. Variations during manufacturing are influential enough to prevent preciseness in measured capacitance levels. The most problematic variations include improperly distancing plates from one another, improperly orienting them relative to each other or arranging them wrongly on housing containers. Owing to common calibration schemes in devices using the plates, specific capacitance readings cannot be always associated with a specific ink level remaining in the supply item.
Also, capacitance readings correspond typically to a decrease in farads (F) as fluid levels between spaced plates become lower over time. Conversely, refilling fluid leads to higher capacitance readings. Plotting one variable relative to the other usually results in constantly sloped data in graphs, e.g., FIG. 6. However, distinguishing a reading of 8.2 pF from a reading of 8.1 pF does not easily lend itself to knowing an actual height of fluid in a container. While the latter value can be generally acknowledged as corresponding to a height of fluid lower than the former value, correlation to a measurement of height in distance units sometimes proves challenging. Correlation of fluid to an absolute height in distance units above a floor of a container is equally challenging. Similarly, the lowering of farad (F) values with the consumption of liquid is an expected result over time. Little knowledge is learned from measuring decreased capacitance values other than assuming the depletion or lowering of fluid. It would be useful, on the other hand, to know exact heights of fluid in distance units, despite uncertainties in manufacturing variances and calibration techniques. It would be useful further to know height milestones, such as when a container is exactly half full or a quarter empty, for example.
Accordingly, a need exists in the art to improve fluid level detection in supply items of imaging devices, especially when involving capacitance measurement techniques. The need extends not only to improving accuracy, but to translating capacitance readings into beneficial heights of fluid. Simplicity of design is still a further recognized need as is eliminating tolerance variability in manufacturing. Economic advantage is still another consideration. Additional benefits and alternatives are also sought when devising solutions.